You get scheduled shifts for when you are supposed to work. Sometimes you’re a scheduled as an On-Call. That means that you have to call in and ask if they need to work that day. You might have to work if someone who was scheduled to work that day called out or if it’s going to be really busy. Being On-Call sucks. If you assume you won’t get called in and make plans, then you’ll be called in. If you plan on working, they won’t need you. It’s a catch-22. You can’t win.